1. Field of the Invention
The present invention relates to a device for measuring the radiation temperature of a melt in vacuum, which uses a pyrometer.
2. Description of Related Art
In measuring the temperature of molten samples, in particular of metals, in a vacuum, there is a problem that sample material in a vapor state will be deposited on the wall of the vacuum chamber or the window of the pyrometer. In this case, the member, initially permeable to the radiation, will quickly become impermeable to the radiation because of the evaporation, and will be unfit for further pyrometric use.
From Japanese Patent 60-57224 A, in: Patents Abstract of Japan, Vol. 9 (1985), No. 186 (117 P 377), a device for measuring the radiation temperature of a melt in vacuum as known wherein the radiation thermometer is shielded against vaporous matter by a perforated screen with a central passage. The shielding cannot be complete, since the pyrometer is still orientated directly towards the melt and, thus, is still evaporated. Moreover, only a comparatively small amount of the radiation emitted by the melt reaches the pyrometer, due to the shielding, so that the temperature measurement is rather inexact.
It is an object of the present invention to provide a device of the type mentioned above, wherein the pyrometer is reliably protected against an evaporation by the melt
In a first variant of the device of the present invention, an optical grating arrangement is provided between the melt and the pyrometer, which concentrates the radiation coming from the melt onto the pyrometer, whereas the direct path between the melt and the pyrometer, or between the melt and a window disposed before the pyrometer, is blocked. Thus, vapor molecules that escape from the melt at high velocities are kept from getting to the pyrometer or the window arranged before the same on a straight path and from being deposited there. A part of the electromagnetic radiation (heat and light radiation) is deflected by the disperging effect of the optical grating arrangement for radiation focusing. Usually, a pyrometer operates at wavelengths in the magnitude of 1.mu.m to about 3.mu.m. In this case, a grating constant of the optical grating of 50.mu.m will suffice to achieve a sufficient radiation deflection. For the heat radiation, the grating arrangement has the same effect that an optical diffraction grating has for light radiation, whereas, for the molecules of the melt material, the grating arrangement is like a simple grating that absorbs a part of the impinging molecules and lets the other part pass linearly.
Advantageously, the melt, the grating arrangement and the pyrometer are arranged along a straight axis on which a shielding screen is disposed behind the grating arrangement, seen in the direction of the radiation, the radial extension of the screen being smaller than that of the grating arrangement. It is the purpose of the shielding screen to keep the main flow of vapor from the pyrometer.
In an advantageous embodiment of the invention, the grating arrangement is designed such that it deflects the impinging radiation from the optical axis. Such a grating arrangement is particularly suited for two-color pyrometric temperature measurements, wherein pyrometers are provided in an arrangement depending on the deflection of the radiation which depends on the respective wavelengths, on which radiation of different wavelengths impinges. In a grating arrangement that deflects the impinging radiation from the optical axis, the vapor will pass through the grating along the optical axis, whereby the vapor and the radiation used in measuring the temperature are separated behind the grating.
The grating arrangement may be formed of any kind of grating in which the spaces between the grating lines are open and permeable to matter. For instance, a grating of parallel thin wires or a cross grating of thin crossing grating rods may be used. Preferably, a zone plate of numerous rings arranged in one plane is used, because zone plates have image-reproducing properties.
According to a second variant, a grating arrangement that is partly permeable to matter is disposed between the melt and the pyrometer, in front of which a direction-selective device is arranged in the path of the radiation. Radiation from that portion of the melt, onto which the direction-selective device is directed, will reach the grating arrangement via the direction-selective device. The grating arrangement may be the zone plate described above, having image-reproducing properties and, depending on its configuration, effecting a deflection of radiation, whereby the radiation and the flow of vaporous melt material are separated. Advantageously, a non-reproductive diffraction grating is used as the grating arrangement; in this case, the radiation diffracted at the diffraction grating is employed for the temperature measuring.
In an advantageous embodiment of the second variant of the present invention, the direction-selective device consists of a bundle of parallel tubes, the interiors of which are preferably provided such that they absorb light, e.g. by means of a suited coating, in order to avoid reflections of light. This is necessary, because light reflections at the inner walls of the tubes can interfere with pyrometric temperature measuring, since reflections can be wavelength-selective so that the radiation reaching the pyrometer does not have the spectral range of the radiation emitted by the melt. Reflections within the tubes or the channels of the direction-selective device should also be avoided because the reflection properties may vary in dependence on the amount of material deposited, for which reason the accuracy of measurement changes with an increasing operation time.
Advantageously, the direction-selective device consists of a plurality to perforated screens, diaphragms, blinds or the like arranged behind one another in the path of radiation, each having at least one hole. The relative position of the individual perforated screens is selected such that all holes or groups of holes of the perforated screens are in flush. Depending on the number to holes in each screen, the perforated screens provide "passages" between the melt and the grating arrangement. Between the individual perforated screens, the passages are open at the sides. No light reflections can occur in these laterally open passage areas; on the other hand, no scattered light can enter the passages at the open areas, either. Furthermore, the production of perforated screens by etching the holes into a film of material is rather simple. Advantageously, the screens consist of a lightabsorbing material.
Preferably, a shielding screen with a hole is disposed between the grating arrangement and the pyrometer, the portions of the screen that are impermeable to radiation being arranged in a linear projection of the aligned holes of the perforated screens, i.e. in a linear projection of the channels or tubes of the direction-selective device. The shielding screen intercepts vapor particles after these have passed the channels or the perforated screens and the diffraction grating. In this way, the successively arranged pyrometer is even more reliably protected from vaporous melt material. If the temperature measuring uses the light diffracted at the non-reproductive diffraction grating, the subsequent pyrometer may be arranged such that it is not evaporated. If a diffraction grating is used as the grating arrangement, the optical system of the pyrometer should be designed such that it reproduces the plane of the diffraction grating on the detectors of the pyrometer. Since a non-reproductive diffraction grating, other than e.g. a reproductive diffraction grating (zone plate), does not concentrate radiation in one direction, like e.g. an optical lens, it is the purpose of the optical system of the pyrometer to concentrate the radiation (on the light detectors). The light may also be concentrated by means of optical fibers or a mirror optic on which the diffracted light impinges and which transmit it to the light detectors. The optical fibers are a low-cost solution to the problem of the focusing and the concentration of radiation towards detectors of the pyrometer.
Advantageously, the holes in the perforated screens are concentric slits of narrow width. The configuration and arrangement to the curved slits allows to select the range of the sample surface to be examined, the radiation of which range is allowed to pass to the diffraction grating. The slits may be concentric annular apertures extending over 360.degree. and being interrupted by radial ridges provided for mechanical stability and for holding together the also annular opaque regions of the perforated screens.
In order that a large quantity of light reaches the pyrometer, it is necessary to generate "many light beam edges". In other words: a plurality of "radiation beams" should impinge on the diffraction grating, which is obtained by providing a corresponding number of holes per perforated screen and/or by designing the holes as concentric annular slit openings.
The radial distance of the concentric slits increases advantageously with an increasing proximity of the perforated screens to the diffraction grating. Accordingly, the concentric slits of the perforated screen farthest from the diffraction grating, i.e. the first screen to the melt, have a lesser radial distance than the concentric slits of the perforated screen arranged immediately before the diffraction grating. Since the holes of different screens are in alignment, channels or passages are formed that resemble cone surfaces. By arranging a plurality of concentric annular slits of different distances to the center per screen, cone surface-like passages of different diameters are formed, the tips of all cones coinciding in the observation point of the melt. The width of the annular slits determines the thickness of the passages. The configuration of the screen package described above allows to sense light coming from a limited portion (observation point) of the melt in a maximum solid (dihedral) angle.
Advantageously, the distances between neighboring perforated screens, and between the last perforated screen and the shielding screen are displaceable by the same factor. Advantageously, this is accomplished by arranging the entire package of perforated screens in the path of radiation so as to be longitudinally displaceable. The displaceability of the screens and the simultaneous change in distance by the same factor allow an adjustment of the evaporation protection consisting of the grating arrangement, the direction-selective device and, possibly, the shielding screen, to different distances from the surface of the melt. A relative horizontal displacement of the perforated screens, i.e. a displacement rectangular to the optical axis, by amounts which each are proportional to the distance of the perforated screens from the shielding screen, advantageously permits a horizontal shifting of the observation point.
In order to minimize both the changes in the dimensions of the openings in the diffraction grating and the changes in the optical properties of the grating caused by the evaporation, it is feasible to have the vapor jet encounter the diffraction grating substantially vertically. This can also be achieved by a corresponding arrangement of the diffraction grating. It as a particularly advantageous arrangement to provide the diffraction grating as a spherical surface with the melt observation point as the center of the sphere. The individual perforated screens are advantageously also provided as sections to a spherical surface, the centers of all spheres also coinciding with the melt observation point.
In order to obtain a maximum condensation coefficient of the particles, the perforated screens, the grating arrangement and the shielding screen are cooled by a cooling device.
The device of the present invention may be implemented for the pyrometry of substances evaporating in vacuum, be they solid or liquid, for instance to measure the temperature of a melt or a substance sublimating in vacuum.